Castoroides, or giant beaver, is an extinct genus of enormous beavers that lived in North America during the Pleistocene. C. leiseyorum and its northern sister speciesCastoroides ohioensis, were the largest beavers to ever exist.

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Castoroides, also known as giant beavers, were much larger than modern beavers. Their average length was approximately 1.9 m(6 ft), and they could grow as large as 2.2 m(7 ft). The weight of the giant beaver could vary from 90 kg(198 lb) – 125 kg(276 lb). This makes it the largest known rodent in North America during the Pleistocene and the largest known beaver.[2]

The hind feet of the giant beaver were much larger than in modern beavers, while the hind legs were shorter. The tail was longer but may have been narrower.[1] However, because soft tissues decay, it is not known whether its tail resembled the tails in modern beavers, and it can only be assumed that its feet were webbed like in modern species.[3][2] The skull structure of the giant beaver shows that it presumably participated in extended underwater activity, thanks to the ability to take in more oxygen into its lungs.[3]

One of the defining characteristics of the giant beaver were their incisors, which differ in size and shape than those of modern beavers. Modern beavers have chisel-like incisor teeth for gnawing on wood, while the teeth of the giant beaver were bigger and broader, and grew to about 15 cm (6 in) long.[4] These incisors were not as efficient at cutting wood, therefore it is possible that the giant beaver did not construct dams.

One other major difference between the giant beaver and the modern beaver is that the size of its brain was proportionally smaller. As a result, giant beavers may have had inferior interactions in its environment as well as less complex patterns of thoughts and behavior.[5]

The Casteroides fossils were discovered in 1837 in a peat bog in Ohio,[4] hence its species epithet ohioensis. Catalogue no.1195, Mus. North. Ind. Hist. Soc. Well- preserved skull of Castoroides ohioensis but with the mandibles lost, both zygomatic arches missing, and the facial portions of the maxillae broken away; dental series complete and in good condition.[7] Castorrides had cutting teeth up to 15 cm-long with prominently-ridged outer surfaces.These strong enamel ridges would have acted as girders to support such long teeth. Further, the deep masseteric fossa of the lower jaw suggests a very powerful bite. Perhaps their teeth could have acted as both wood-cutters and gouges.There is no clear evidence that the giant beaver felled trees or built dams, but a possible lodge was discovered near New Knoxville, Ohio about 1912. Part of a giant beaver skull and the lodge were located in a peaty layer surrounded by loam.[8] In Ohio, there have been claims of a possible giant beaver lodge four feet high and eight feet in diameter, formed from small saplings.[4] The recent discovery of clear evidence for lodge building in the related genus Dipoides indicates that the giant beaver probably also built lodges.[9]

Fossils of Casteroides are concentrated around the midwestern United States in states near the Great Lakes, particularly Illinois and Indiana, but specimens are recorded from Alaska and Canada to Florida. In Canada, fossils of this species are commonly found in the Old Crow Basin, Yukon, and single specimens are known from Toronto, Ontario and Indian Island, New Brunswick. A hitherto overlooked 1891 record of a Casteroides skull from near Highgate, Ontario is the earliest for Canada.[10] In Old Crow region, Casteroides fossils occur in deposits of the Sangamonian interglacial.[11]

The discovery of giant beaver remains in New Brunswick add significantly to the Quaternary terrestrial mammal fauna of New Brunswick and suggest that the terrestrial fauna was probably richer than earlier evidence indicated. The know North American distribution of giant beaver is not significantly changed by this occurrence.[1][12] Specimens from Florida have been placed in a subspecies, Castoroides ohioensis dilophidus, based on differences in premolar and molar features.[13] Castoroides, the giant beaver, is recorded from 25 Pleistocene localities in Florida, 23 of Rancholabrean age, one possibly of Irvingtonian age, and one of late Blancan age.[14]

Fossils of the older species, C. leiseyorum, from Florida are from 1.4 Mya, while fossils of the younger species, C. ohioensis, from Toronto, Ontario, and the Old Crow Basin, Yukon Territory, are 130,000 years old, but Casteroides may have died out about 10,000 years ago, along with several other American species, such as mammoths, mastodons, and ice-age horses. Although no AK–YT fossils of the Castoroides have been recorded after the Last Glacial Maximum (LGM) about 18 kyr BP.[21]

The extinction of the giant beaver may have been caused by ecological restructuring at the end of the Pleistocene.[22] The arrival of humans in the Americas could have been a factor, but there is no evidence that humans hunted the giant beaver.[4] It was one of the abundant Pleistocene megafauna—a wide variety of very large mammals that lived during the Pleistocene.

Evidence from the lower Hudson Valley of New York shows that megafaunal populations collapsed many centuries in advance of the extinction event . At four localities (Otisville, Hyde Park, Binnewater Pond, and Pawelski Farm), the onset of the megafaunal population decline predates the Younger Dryas (at 12.9 cal ka BP). However, extinct megafauna (e.g., Castoroides) are securely dated to the very latest Pleistocene in nearby localities, indicating that population collapse preceded the actual extinction event. The timing of the population decline is of particular interest in that it predates a number of possible extinction mechanisms, including Younger Dryas climate change, the arrival of Clovis people in North America, and the proposed extra-terrestrial impact event. Sporormiella records from a sediment core recovered from Appleman Lake, Indiana show a pronounced decline in megafaunal populations during the Bølling–Allerød interstadial from 14.8 to 13.7 cal ka BP. Once again, this decline precedes the extinction for many taxa, indicating a population collapse well before the extinction event. A Nutrient decelerating model suggests that as forage quality declines, herbivores must preferentially rely on those species with the highest nitrogen concentrations. Preferential feeding on plants with higher nitrogen concentrations tends to reduce plant litter quality, further limiting nitrogen availability in the soil. Over time, these feedback mechanisms decrease nitrogen availability, resulting in reduced forage production, decreased nitrogen concentrations, and reduced mammalian herbivore biomass.[23]